Transdiagnostic factors in symptoms of depression and post-traumatic stress: a systematic review

The current systematic review sought to identify quantitative empirical studies that focused on the transdiagnostic factors of intolerance of uncertainty, emotional dysregulation and rumination, and their relation with depression and post-traumatic stress disorder (PTSD). The overall research aim was to examine the relationship between these transdiagnostic factors and their relation with depression and PTSD symptoms. The systematic review was conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines. Out of the 768 articles initially identified, 55 met the inclusion criteria for the current review. The results determined that intolerance of uncertainty is indirectly related to depression and PTSD symptoms, mainly through other factors including emotion dysregulation and rumination. Additionally, emotional dysregulation is a significant predictor of both depression and PTSD symptoms. Rumination is a robust factor related to depression and PTSD symptoms, this relationship was significant in cross-sectional and longitudinal studies. This review provides evidence on the transdiagnostic factors of intolerance of uncertainty, emotional dysregulation and rumination in the relationship with depression and PTSD symptoms.


Epidemiology prevalence and comorbidity
Depressive, anxiety, and substance use disorders are among the leading contributors to the global disease burden (Whiteford et al., 2013). Depression is one of the most common mental disorders, with an estimated prevalence of 4.4% for the past year (WHO, 2017). Mental disorders rarely occur in isolation, particularly major depression disorder (MDD) has shown elevated comorbidity rates among different psychopathologies (Watson, 2009). This pattern of comorbidity can be seen with post-traumatic stress disorder (PTSD). Epidemiological studies indicate that around 80% of patients with PTSD may have comorbid disorders including depression, anxiety, or substance abuse (Foa et al., 2000). Among individuals who meet the criteria for PTSD diagnosis 48% to 55% also meet the criteria for MDD (Elhai et al., 2008;Kessler et al., 1995). This elevated comorbidity rate persisted even after controlling for overlapping symptoms (Brady et al., 2000;Elhai et al., 2008;Grubaugh et al., 2010).
Individuals with comorbid symptoms of depression and PTSD, when compared to individuals with only one type of symptom group, report lower levels of functioning, more severe symptoms, and poorer treatment outcomes (Brady et al., 2000). The presence of comorbidity entails difficulties for their diagnosis, clinical management, greater probabilities of resistance to treatment, recurrence, and use of health resources (Clark et al., 2012). However, despite the evidence of elevated comorbidity rates and considerable amount of symptom overlaps between depression and PTSD (Brady et al., 2000), the study of vulnerability factors has generally focused on their individual contribution to psychopathology (Hong & Cheung, 2015). However, less is known about basic psychological processes that may contribute to this comorbidity. Thus, it is important to understand what psychological factors contribute to the co-occurrence of depression and PTSD.

Transdiagnostic factors
Transdiagnostic models of psychopathology emerged in response to the evidence of the lack of specificity of vulnerability factors to explain individual mental disorders, as well as the overlap of symptoms of presumably categorically different disorders. A transdiagnostic approach to psychopathology aims to understand different mental disorders or consistent groups of mental disorders based on similar cognitive, behavioral, and physiological processes involved in their etiology (Sandín et al., 2012). Moreover, this approach has allowed generating a framework with a flexible conceptualization that facilitates the understanding of the comorbidity patterns present in psychopathology, based on common variables associated with the development, maintenance, and treatment (Aldao, 2012). The study of common vulnerability factors among disorders will make it possible to understand comorbidity, better define current diagnostic classification systems, and design appropriate treatment strategies for a wide range of disorders. Several vulnerability factors have shown associations with multiple psychopathological symptoms. Particularly, for symptoms of depression and PTSD, various factors of a transdiagnostic nature have been identified including: intolerance of uncertainty (Carleton et al., 2012;Gentes & Ruscio, 2011), emotional dysregulation (Sloan et al., 2017), and rumination Olatunji et al., 2013).

Intolerance of uncertainty
Intolerance to uncertainty is the dispositional inability of an individual to withstand the response triggered by the perceived absence of relevant, key or sufficient information (Carleton, 2016). Individuals with high levels of intolerance of uncertainty are more likely to interpret uncertainty negatively (Carleton et al., 2007). Moreover, uncertainty may contribute to maladaptive emotional, cognitive and behavioral processes that are associated with emotional distress, such as general anxiety disorder and panic disorder, (Boswell et al., 2013;Buhr & Dugas, 2009). Intolerance of uncertainty is a two-dimensional construct consisting of the prospective and the inhibitory dimensions. The prospective dimension has an anticipatory cognitive nature and is conceptualized as a desire for predictability of future events. The inhibitory dimension refers to behavioral paralysis and impaired functioning due to uncertainty (Carleton et al., 2007). Evidence from multiple studies indicating substantial associations between intolerance of uncertainty and internalizing disorders, has prompted researchers to conceptualize intolerance of uncertainty as a major transdiagnostic risk factor in these conditions (Shapiro et al., 2020). A recent meta-analysis underscored the transdiagnostic nature of intolerance of uncertainty, finding strong and significant associations between intolerance of uncertainty and symptoms of depression, anxiety, and eating disorders (McEvoy et al., 2019). Intolerance of uncertainty has also been suggested as a vulnerability risk factor for elevated PTSD symptoms (Banducci et al., 2016;Oglesby et al., 2016).

Emotion dysregulation
Emotional dysregulation is a multidimensional construct that involves maladaptive ways of responding to emotions, including (a) lack of awareness, understanding, and acceptance of emotions; (b) the inability to control behaviors when experiencing emotional distress; (c) lack of access to situationally appropriate strategies to modulate the duration and/ or intensity of emotional responses to meet individual goals and situational demands; and (d) unwillingness or reluctance to experience emotional distress as part of seeking meaningful activities in life (Gratz & Roemer, 2004). People who have difficulties regulating their emotions are more likely to engage in maladaptive behaviors such as impulsivity, avoidance, substance use, and other risky behaviors (Weiss et al., 2012). Difficulties with emotional regulation have also demonstrated their transdiagnosis role in a range of psychopathologies including depression , substance use disorder (Wong et al., 2013) and PTSD (Seligowski et al., 2014). Specifically,  examined the specificity of cognitive emotional regulation strategies from a transdiagnostic perspective. They found that dysfunctional emotional regulation strategies were associated with indicators of psychopathology symptoms (eating, depressive and anxious), this association remained regardless of the specific disorder.

Rumination
Rumination is defined as repetitive and passive thoughts about negative emotions, their possible consequences and causes (Nolen-Hoeksema, 2000;Nolen-Hoeksema et al., 2008). Rumination prevents active problem solving to change the situations causing these negative emotions. Consequently, individuals focus on the problems and their feeling about them rather than taking action (Nolen-Hoeksema et al., 2008). It has been suggested that rumination consists of two components: reflection, which indicates a tendency to focus on one's thoughts in order to find a solution to the problem or situation that distresses the individual, and brooding, which refers to a passive reflection were no action is taken to improve the situation (Treynor et al., 2003). Rumination has been traditionally associated with depressive symptoms and has been considered an important factor in the onset, maintenance, and recurrence of depression (Nolen-Hoeksema et al., 2008;Olatunji et al., 2013). Individuals who engage in ruminative processes report greater levels of depressive symptoms over time (Nolen-Hoeksema, 2000). Rumination has also been identified as a maladaptive cognitive style associated with multiple disorders and symptoms . Particularly, for PTSD symptoms it is considered an important factor that maintains and increases symptom severity (Szabo et al., 2017).

The present review
As previously mentioned, depression and PTSD are highly comorbid disorders with more than half of the people with a lifetime diagnosis of PTSD also meeting the criteria for a diagnosis of MDD (Elhai et al., 2008). This elevated comorbidity rate persisted even after controlling for overlapping symptoms (Brady et al., 2000;Elhai et al., 2008;Grubaugh et al., 2010). Transdiagnostic models and the study of vulnerabilities factors between depression and PTSD symptoms is important for better understanding of comorbidity, improving theoretical models of comorbidity and guiding clinical intervention. Thus, it is crucial to investigate how these factors play a role in the relationship between these highly comorbid disorders. The current systematic review sought to identify quantitative empirical studies that focused on the transdiagnostic factors of intolerance of uncertainty, emotional dysregulation and rumination, and their relation with symptoms of depression and/or PTSD. The overall research aim was to examine the relationship between these transdiagnostic factors and depression and /or PTSD symptoms.

Literature search
The review was conducted in accordance with the Preferred Reporting Items for Systematic Review and Meta-Analyses (PRISMA) guidelines (Liberati et al., 2009). Studies were identified by searching multiple databases including PsychInfo, Medline, Web of Science, and Google Scholar during June and July 2021. Keywords/Search terms included various combinations of 'intolerance of uncertainty', 'emotion regulation', 'emotional dysregulation', 'rumination', 'depression', and 'posttraumatic stress disorder'.

Inclusion and exclusion criteria
Eligible empirical studies were included if they met the following inclusion criteria: (1) Empirical quantitative research reports published in a peer reviewed journal (2) Written in English or Spanish language, (3) Outcome measures related to depression or PTSD symptoms or diagnosis. Studies were excluded if the following criteria was met: (1) No depression or PTSD outcome measure, (2) No rumination, intolerance of uncertainty or emotional dysregulation measure, (3) Not depression PTSD comorbidity, (4) Children or adolescent sample, (5) Insufficient or inappropriate data reported ( Fig. 1, Table 1).

Intolerance of uncertainty and depression
Eleven studies examined the relation between intolerance of uncertainty and depression symptomatology. Empirical evidence indicates that intolerance of uncertainty is a significant predictor of depression symptoms, even when anxiety symptoms where accounted for (Barry et al., 2019;Del Valle et al., 2020). Furthermore, the relationship between intolerance of uncertainty and depression was indirect and partially mediated by other risk factors, including worry (Dar et al., 2017;Swee et al., 2018), trait anxiety (Swee et al., 2018), rumination (Huang et al., 2019), and fear of COVID-19 (Voitsidis et al., 2021). Two risk factors suggested a complete mediation of the effects of intolerance of uncertainty on depression symptoms: maladaptive metacognitive beliefs (Chen et al., 2021) and rumination over two months (Huang et al., 2019). Lower-order dimension analysis of intolerances of uncertainty and a two-factor model of depression indicated that a general factor of intolerance of uncertainty was related to cognitive and affective/somatic factors of depression symptoms (Saulnier et al., 2019). However, the Inhibitory dimension of intolerance of uncertainty was only related to the cognitive factor of depression, while the Prospective dimension was not related to any of these depression factors. Worry was the only risk factor that moderated the relationship between intolerance of uncertainty and symptoms of depression, consequently high levels of worry heightened the association between intolerance of uncertainty and symptoms of depression (Dar et al., 2017). An experimental treatment study determined that reductions in intolerance of uncertainty were associated with reductions in repetitive negative thinking, but not with depression symptoms (McEvoy & Erceg-Hurn, 2016).

Intolerance of uncertainty and post-traumatic stress
Six studies that examined the relation between intolerance of uncertainty and PTSD symptoms. This evidence indicates that intolerance of uncertainty is a significant predictor of PTSD symptoms. These result remained significant even after covarying for other PTSD risk factors including: rumination, neuroticism, and worry (Boelen et al., 2016), anxiety sensitivity (Oglesby et al., , 2017 and negative affect (Oglesby et al., 2017).
When the relation between intolerance of uncertainty and PTSD symptoms DSM-V clusters was examined, there was a significant association to the avoidance, hyperarousal and emotional numbing symptom clusters (Oglesby et al., 2017). Dimension analysis of intolerances of uncertainty indicated that only the inhibitory dimension was associated with PTSD symptoms (Boelen, 2019;Boelen et al., 2016). Further, a decrease in overall and inhibitory dimension was associated with decreases in PTSD severity and at the symptom cluster level. However, the prospective dimension only associated with changes in the re-experiencing,       avoidance, and hyperarousal clusters (Badawi et al., 2021). Finally, mediation analysis determined that intolerance of uncertainty was not a significant mediator between distress tolerance and PTSD symptoms (Raudales et al., 2020).

Emotion dysregulation and depression
We identified 8 studies that assessed the relation between emotion dysregulation and depression symptoms. Results indicated that emotion dysregulation was positively associated with depressive symptoms, even when controlling for baseline mindfulness (Diehl et al., 2020). Furthermore, emotional dysregulation partially and significantly mediated the relationship between depression and various factors related to depressive symptomatology, including, cumulative adversity (Abravanel & Sinha, 2015), anxiety sensitivity (Ouimet et al., 2016), maladaptive beliefs about emotions (Ouimet et al., 2016), and attachment style (Pickard et al., 2016). One study examining which emotion regulation skill mediated the association between self-compassion and depression, determined that only the ability to tolerate negative emotions was a significant mediator (Diedrich et al., 2017). Likewise, expressive suppression was the only emotion regulation strategy that mediated the relationship between mental toughness and depressive symptoms (Mutz et al., 2017). Khakpoor et al. (2019) conducted an experimental study that examined how the Unified Protocol for transdiagnostic treatment of emotional disorders reduced depression in patients through improvement in emotion regulation. Emotion regulation, particularly difficulty engaging in goal-directed behavior and lack of emotional clarity, predicted most of the variance in depression scores. Furthermore, mediation analysis concluded that emotion regulation can be considered a mediating factor and a predictive of outcomes of transdiagnostic treatment based on the Unified Protocol (Khakpoor et al., 2019). Finally, in the context of COVID-19, loneliness predicted higher depressive symptoms and depressive symptoms predicted higher loneliness one month later. This relationship was not mediated by emotion regulation difficulties. Instead, emotion regulation difficulties and depressive symptoms were reciprocally related (Groarke et al., 2021).

Emotion dysregulation and post-traumatic stress
Seven studies assessed the relation between emotional dysregulation and PTSD symptoms. Longitudinal assessments found that emotional dysregulation was significantly associated with the probability of developing PTSD symptoms 4 months later (Fujisato et al., 2020) and at 12 months (Pencea et al., 2020), even when controlling for baseline PTSD, trauma exposure, and depressive symptoms. Likewise, emotion dysregulation predicted PTSD symptom severity at 3 months following exposure to a traumatic event, even after covarying for baseline PTSD symptoms and other risk factors such as, trauma type, childhood adversity, trauma exposure, lifetime trauma exposure, age, gender, race, and ethnicity (Forbes et al., 2020). Mediation analysis determined that emotional regulation mediated the effects of trauma type, particularly sexual assault, and PTSD symptoms (Raudales et al., 2019) and negative affect and PTSD and MDD (Post et al., 2021). When the relation between emotion dysregulation and PTSD symptom clusters were examined, emotional dysregulation only predicted avoidance and hyperarousal symptom severity even after covarying the number of trauma types and negative affect (O'Bryan et al., 2015). Difficulties in emotion regulation subscales analyses determined that difficulties in impulse control predicted re-experiencing, avoidance, and hyperarousal, whereas lack of emotion regulation strategies and emotional clarity were uniquely associated with emotional numbing symptoms .

Rumination and depression
Thirteen studies examined the relation between rumination and depression symptoms. Results from longitudinal associations indicated that rumination predicts depressive symptoms and that depressive symptoms predict rumination at 4-year follow-up (Whisman et al., 2020). Likewise, a correlation of stressful events and rumination predicts depressive symptoms prospectively (3 and 15 months follow-up), even when controlling for baseline depressive symptoms (Vanderhasselt et al., 2016). However, one longitudinal study found that rumination (brooding and reflection subscale) did not predict depressive symptoms one year later, when compared to trait mindfulness and while controlling for baseline depression symptoms (Schut & Boelen, 2017). Mediation analysis determined that rumination was a significant mediator between maladaptive perfectionisms (De Rosa et al., 2021), gratitude (Liang et al., 2020), non-judge mindfulness component (two years after) (Petrocchi & Ottaviani, 2016), emotional clarity (Vine & Marroquin, 2017), overgeneral autobiographical memory (Liu et al., 2017), and negative affect (Iqbal & Dar, 2015) and depression symptoms. However, rumination was not a significant mediator of the relationship between negative affect and depression, when taking into account cognitive fusion in the mediation model (Costa et al., 2018), contradicting previous research (Iqbal & Dar, 2015). Subscale analysis of rumination indicated that brooding mediated the relationship between overgeneral autobiographical memory (Liu et al., 2017), perfectionism (Senra et al., 2017), selfcompassion (Bakker et al., 2018) and depression symptoms. Further, brooding mediated the effect of various personality traits such as: neuroticism, extroversion, conscientiousness and openness on depressive symptoms, while reflection mediated the effects of neuroticism, extroversion and openness on depressive symptoms (Lyon et al., 2020).

Rumination and post-traumatic stress
Ten studies examined the relation between rumination and PTSD. Repetitive and anticipatory rumination moderated the relationship between PTSD and MDD symptoms (Roley et al., 2015). While, mindful attention moderated the relations between rumination and PTSD symptoms total, as well as all its symptom clusters (Viana et al., 2017). Interpersonal trauma moderated the relationship between baseline rumination and 1-month follow-up PTSD symptoms, even after covarying for negative affect, and number of previously experienced traumas (Preston et al., 2021). Longitudinal association determined pre-trauma depression severity and trait rumination predicted the onset of PTSD during a 4-year follow-up (Spinhoven et al., 2015). PTSD symptom clusters analysis indicated that rumination was significantly related to all symptom cluster (Seligowski et al., 2016). Mediation analysis determined that that hostility (Mathes et al., 2020), cognitive appraisal of the traumatic event (Spinhoven et al., 2015) and deliberate rumination (García et al., 2018) mediated the relationship between rumination and PTSD symptoms. Furthermore, rumination also acted as a mediator between emotional dysregulation and PTSD symptom severity (Pugach et al., 2019). Finally, experimental avoidance and rumination were compared in groups with and without PTSD, in the group with PTSD rumination was significantly higher than those without PTSD (Basharpoor et al., 2015). Brooding rumination and depressed-related rumination were related with higher level of PTSD (Wu et al., 2015).

Discussion
The current systematic review sought to identify quantitative empirical studies that focused on the transdiagnostic factors of intolerance of uncertainty, emotional dysregulation and rumination, and their relation with depression and/or PTSD. This review identified 55 studies that reported the association between the transdiagnostic factors of interest in depression and PTSD symptoms. Intolerance of uncertainty was a consistent significant predictor for both depression and PTSD symptoms, as suggested by other authors (McEvoy et al., 2019;Shapiro et al., 2020). This association persisted after controlling other wellknown risk factors (e.g., negative affect, worry, neuroticism, and anxiety sensitivity), however anxiety sensitivity was the only variable that covaried in both depression and PTSD symptoms. This may be explained by the evidence that suggests anxiety sensitivity is broadly related to a range of internalizing disorders particularly distress disorders such as depression, GAD and PTSD (Naragon-Gainey, 2010). Mediation analysis suggested that intolerance of uncertainty and depression are indirectly related. This relationship turned out to be partially mediated by other risk factors including worry and trait anxiety. Likewise, rumination had a partial mediation role in cross sectional studies, whereas longitudinally rumination fully mediated the relation between intolerance of uncertainty and depression. Therefore, individuals with high levels of intolerance of uncertainty ruminate over time as an attempt to cope with negative emotions, meanwhile increasing the risk of developing depressive symptoms (Huang et al., 2019). Only one study conducted a mediation analysis taking into consideration intolerance of uncertainty as a mediator between emotional dysregulation and PTSD symptoms, however this relation was not significant. This result suggests that intolerance of uncertainty may be indirectly related to PTSD symptoms possibly through other risk factors, in a similar way as found in depression symptoms. Lower-order dimension analysis of intolerances of uncertainty indicated that the inhibitor dimension was only related to cognitive factors of depression and PTSD clusters avoidance and hyperarousal. This result supports the notion that the inhibitory dimension refers to impaired functioning due to uncertainty. While the prospective dimension was not related to any individual factors of depression, it was related to avoidance, hyperarousal and re-experiencing PTSD symptoms. Particularly, this dimension has an anticipatory cognitive nature and is conceptualized as a desire for predictability of future events.
Emotional dysregulation has been found to relate to depression as well as many other psychiatric symptoms including PTSD . In this systematic review emotional dysregulation was a significant predictor of both depression and PTSD symptoms. For depression symptoms this result was significant also after covarying for mindfulness. In relation to PTSD symptoms, longitudinal analyses determined that emotional dysregulation remained a significant predictor also after covarying other well established PTSD risk factors including, baseline PTSD symptoms, depressive symptoms, trauma type, trauma exposure, childhood adversity, lifetime trauma exposure, age, gender, race, and ethnicity. Furthermore, emotional dysregulation has a mediating effect between several risk factors and depression or PTSD symptoms such as, cumulative adversity, anxiety sensitivity, attachment style, maladaptive beliefs about emotions, sexual assault and negative affect. PTSD symptom cluster analysis indicated that emotional dysregulation predicted avoidance and hyperarousal symptoms but not re-experiencing, even when covarying for number of trauma types endorsed and negative affect. Specific associations between facets of difficulties in emotion regulation and PTSD symptom clusters determined that difficulties controlling impulses while distressed were associated with reexperiencing, avoidance, and hyperarousal symptoms. Therefore, individuals with PTSD symptoms may engage in maladaptive behaviors to help regulate negative emotions such as impulsivity, avoidance, substance use and other risky behaviors (Weiss et al., 2012). Moreover, lack of effective emotion regulation strategies and lack of emotional clarity were related only to emotional numbing PTSD symptoms. This result is also consistent with findings that emotional regulation strategies are related to depression symptoms such as, loss of interest, feeling emotionally numb, and the inability of experiencing positive emotions .
Rumination is an important factor related to the maintenance and exacerbation of depression and PTSD symptoms (Olatunji et al., 2013;Szabo et al., 2017). Longitudinal studies determined that rumination predicted both depression and PTSD symptoms at 4-year follow-up, even after accounting for the effects of baseline depressive symptoms. It should be noted that depression symptoms and rumination influenced one another bidirectionally, that is rumination predicts depressive symptoms and depressive symptoms predict rumination. Rumination has been found to mediate the relationship between depression and several factors related to depression symptoms including, maladaptive perfectionism, gratitude, overgeneral autobiographical memory, mindfulness, emotional clarity, and negative affect. These findings support the theoretic notion that rumination maintains distress, especially depression, through a variety of mechanisms (Nolen-Hoeksema et al., 2008). Evidence for PTSD symptoms indicated that rumination only acted as a mediator between emotional dysregulation and PTSD symptom severity. However, the relationship between rumination and PTSD symptoms was mediated by other factors such as hostility, cognitive appraisal of the traumatic event, and deliberate rumination. PTSD symptom clusters analysis determined that rumination was significantly related to all symptom clusters, which highlights the importance of rumination in individuals who have been through a traumatic event and who may be at greater risk of developing PTSD symptoms. These studies also found that mindfulness and interpersonal trauma moderated of the relationship between rumination and PTSD symptoms. Consequently, interpersonal trauma enhances the association between rumination and PTSD symptoms, while mindfulness weakens this relation. Furthermore, rumination moderated the relationship between PTSD and MDD symptoms, this implies that higher levels of rumination in individuals with PTSD are more likely to also have greater depressive symptoms.

Limitations
This systematic review had a number of limitations that should be taken into account when interpreting the results and addressed in future research. First, even though studies in Spanish were included in an attempt broaden the scope, the majority of the studies were conducted in North American and European countries. Furthermore, taking into consideration the evidence that culture may play a significant role in the manifestation of psychopathology (Rathod, 2017), the results of this study may have limited generalizability to other cultural contexts. Thus, there is a compelling need for future research in other regions of the world to better understand the impact of culture on psychopathology. Second, although elevated comorbidity rates between depression and PTSD symptoms are well known, only two studies were found that addressed comorbidity in these disorders. Future research should aim to investigate factors related to the comorbidity between depression and PTSD symptoms. Third, another significant limitation was that most of studies used crosssectional samples, making it difficult to determine a causal inference. Additionally, most of the studies were from community-based samples which are more likely to present low to moderate symptom levels, thus some studies may have underrepresented symptom levels. This review used a systematic approach. Conducting a meta-analysis on the data would be an interesting future step, to get a more comprehensive understanding of the overall effect across the transdiagnostic factors. Additionally, future studies will need to employ quality assessment of eligible studies using quality assessment tools. Finally, this study relied on questionnaires to assess the variables of interest. Future research should consider incorporating additional measures, such as biomarkers or objective assessments, to provide a more comprehensive and objective understanding of the disorders being studied. Future research should consider incorporating additional measures, such as biomarkers or objective assessments. For example, recent studies have demonstrated that near-infrared spectroscopy (fNIRS) can effectively identify diminished hemodynamic response in major depressive disorder patients, thus providing an objective assessment of depression (Husain et al., 2020;Li et al., 2022). Furthermore, fNIRS has shown potential in identifying brain markers associated with putative symptoms of PTSD (Balters et al., 2021).

Conclusion
This review provides evidence that the transdiagnostic factors of intolerance of uncertainty, emotional dysregulation and rumination are consistent significant predictors for both depression and PTSD symptoms. Particularly, intolerance of uncertainty is indirectly related to depression and PTSD symptoms through other factors including emotion dysregulation and rumination. Meanwhile, emotional dysregulation is a significant predictor of both depression and PTSD symptoms. Rumination is a robust factor related to depression and PTSD symptoms, this relationship was significant in cross-sectional and longitudinal studies. Both emotional dysregulation and rumination can mediate the relationship between several risk factors including intolerance of uncertainty and depression and PTSD symptoms. This study provides insight into understanding these factors in the onset and maintenance of depression and PTSD symptomatology. Consequently, contributing to the evidence that allows the development of theoretical and empirically supported transdiagnostic models. Which may ultimately promote the development of cost-effective treatments that targets underlaying mechanism or factors, thus promoting preventative alternatives for multiple disorders.